The present invention relates to optical fiber telecommunication systems and, in particular, to an apparatus and method of manufacturing optical filtering devices employed in such telecommunication systems.
An optical fiber used in an optical device, such as a wavelength-division multiplexing/demultiplexing (WDM) device in a telecommunication network is typically held in an optical collimator subassembly, which facilitates the alignment and collimation of a multi-wavelength light beam being transmitted through the optical fiber. The collimated light beam exiting the optical fiber is optically transmitted to some type of an optical component, such as, an optical filter or an optical isolator or a grating element in the optical device. The specific type of optical component coupled to the output side of the collimator depends on the specific application. For instance, a WDM device of an optical amplifier system employs optical filters that are precisely aligned and coupled with such collimators in the system. A problem with coupling a light beam into an optical fiber of an optical component in a WDM device is that the light passing through the fiber and emanating from an output end of the fiber immediately begins to disperse, thus, resulting in considerable light signal losses. Accordingly, in order to avoid significant signal loss and to promote maximum signal transfer of an optical signal from one optical fiber to another optical fiber of an optical device or system, a collimating lens is provided, usually at the output end of each optical fiber. A collimator subassembly can have any number of optical ports. The term xe2x80x9cportsxe2x80x9d refers to the total number of optical fiber pigtails contained within a particular optical collimator subassembly. For instance, a one-port collimator subassembly (shown in FIG. 1) has a total of one optical fiber pigtail, whereas, a two-port/dual-port optical collimator subassembly has a total of two optical fiber pigtails, and further, a three-port optical collimator subassembly has a total of three optical fiber pigtails and so on. Similarly, the term xe2x80x9cmultiple-portxe2x80x9d refers to a collimator subassembly having two or more optical fiber pigtails. Typically, a one-port collimator subassembly comprises a glass ferrule or tube, which is disposed at an input end of an insulating glass tube. The ferrule includes an optical fiber pigtail, where a stripped end of the optical fiber is disposed within an axial channel of the ferrule. The collimator subassembly further includes a collimating lens that collimates the light emitted from the optical fiber into parallel rays. The collimating lens is disposed at an opposite output end of the insulating glass tube, adjacent to an output end of the ferrule. A slight gap is left between the output end of the ferrule and an input end of the collimating lens. The output end faces of both the optical fiber pigtail and the ferrule are cleaved, ground and polished at a predetermined angle to prevent back reflections along the optical axis. A type of collimating lens employed in a collimator subassembly is a GRaded-INdex (GRIN) lens, where often the GRIN lens is a cylindrical piece of optical glass with a length that is longer than its diameter. Moreover, the GRIN lens is fabricated to have a radially varying index of refraction that is greater towards the center of the GRIN lens, thus, the GRIN lens is able to produce a focusing effect similar to that of a convex lens. The GRIN lens collimates the light diverging from (or focuses the light to) the smaller core of the optical fiber pigtail held within the ferrule.
The construction of a two-port collimator subassembly is the same as that of a one-port collimator subassembly, except that there are two optical fiber pigtails disposed within the ferrule. An end of each of the fiber pigtails is stripped and is disposed within the ferrule, such that one of the fiber pigtails is aligned in a parallel position to the other fiber pigtail within the ferrule. Similarly, the construction of a three-port collimator subassembly has a ferrule that holds an exposed end of three optical fiber pigtails, with each of the fiber pigtails being disposed within the ferrule, such that each of the fiber pigtails is aligned in a parallel position with respect to the other two fiber pigtails within the ferrule. In this fashion, a variety of single collimator subassemblies having one or more ports can be constructed for a particular application.
Often, such single collimator subassemblies are combined to form an optical filtering device for use in a larger optical system, such as, a wavelength-division multiplexing/demultiplexing (WDM) system. Basically, in a filtering device, an optical component, such as an optical filter is mounted onto a holder and is positioned in between the two collimator subassemblies. The two single collimator subassemblies chosen to construct the filtering device can each have either one-port or multiple ports. Further, the form, function, and placement of the optical component in the filtering device can be selected based upon the particular application of the filtering device. In particular, the optical element can be selected from a group consisting of: an interference filter, a dichroic filter, a thin-film filter, an isolator, a circulator, a gain-flattening filter, a narrow-band filter, a wide-band filter, a passband filter, a band-stripping filter, a grating element, a reflective element, a refractive element, a diffractive element, a liquid-crystal element and an active optical element.
The wavelength of light passed by each filter is highly sensitive to two parameters: 1) the angle of incidence between the filter surface and the light beam, and 2) the position of the beam on the filter""s surface. Hence, the angle at which an optical filter is mounted within such a dual collimator assembly is critical, since the intensity of the signal being transmitted by a filter at a desired wavelength will depend on the position of the signal beam on the filter surface, that is, whether or not the signal beam hits the geometric center of the filter at the specific angle of incidence (AOI). The wavelength that is transmitted through the filter depends on the optical path length traveled by an optical beam through the filter. Moreover, the optical path length depends on the physical thickness of the filter material and the index of refraction of the filter material. When the AOI is varied slightly (as shown in FIGS. 2 and 3), the filter may increase or decrease the intensity of the signal being transmitted at the desired wavelength. Also, as a filter""s AOI is adjusted or varied, the xe2x80x9cpeakxe2x80x9d of the wavelength or the center of the wavelength pulse being transmitted may shift slightly up or down the spectrum, thus, the particular filter may pass a different wavelength than the wavelength at which the center of the filter was characterized. Accordingly, if the AOI is changed, then the intensity may shift, the peak wavelength may shift, or both may shift. At a new AOI, there may be a different peak wavelength at which the maximum intensity of light is transmitted. Thus, each filter can be characterized as having a preferred AOI at which it will transmit a maximum intensity of light at a predetermined center wavelength.
When a WDM device is assembled, all the filters are pre-selected to have compatible properties for the wavelengths being transmitted or reflected. The filters function as a xe2x80x9ckitxe2x80x9d, that is, each of the filters operates at one common angle of incidence at each filter""s geometric center point. Traditionally, in a filtering device, the respective filters are mounted onto associated holders and the filter and holder are manually tilted, that is, the filters are manually aligned with the collimator subassemblies as a light signal is transmitted through the device, until an optimal angle of incidence (AOI) is achieved, where a collimated beam of the light signal hits the geometric center of the optical filter. The filter and holder are then bonded or soldered (secured) into place. As such, the assembly process is long, tedious, labor-intensive and, consequently, overall expensive. It also requires that each of the collimator subassemblies already be assembled and xe2x80x9ctunedxe2x80x9d for the proper alignment of the light going through an output port. A significant problem arises with this process when considering three-port filtering devices. When the AOI of the filter is adjusted or changed, the angle of reflection also changes. Moreover, the change in the angle of reflection dictates where the reflected light signal will hit the ferrule and, therefore, where the bore hole for the output fiber should be located on the input collimator subassembly. Additionally, it is not feasible to manufacture optical filters having a precise AOI, since the multiple dielectric layers of film are not optically flat and can have different configurations.
In light of the foregoing, it is desirable to simplify the manufacturing process for collimator subassemblies and for optical filtering devices and, also, increase line production and reliability with minimal rework. Further, it is desirable to provide a multiple-port collimator subassembly and method for making the multiple-port collimator subassembly that can be easily employed in filtering devices for making a variety of optical multiplexing devices. In addition, it is desirable to provide a manufacturing process that is relatively cost-effective.